This invention relates to an electromagnetic spring-wound clutch, and more particularly to an electromagnetic spring-wound clutch in which the output rotary member can be stopped at any rotational position, and which can be compact in size.
An electromagnetic spring-wound clutch is well known, which comprises an input rotary member, an output rotary member disposed concentrically with the input rotary member, an axially movable collar, a coil spring disposed for gripping engagement with a clutch surface of the input rotary member and engaged at its opposite ends by the output rotary member and the collar, and driving means for electromagnetically driving the collar. As the collar is moved axially of the output rotary member by the driving means, the coil spring comes into or out of gripping engagement with the clutch surface of the input rotary member for causing engagement or disengagement of the clutch.
As known by U.S. Pat. No. 3,637,056, for instance, the above driving means can be formed by an electromagnetic coil, which is adapted to be energized or deenergized to cause generation or extinction of a magnetic flux for magnetic attraction of the collar for engagement or disengagement of the clutch. However, according to this conventional clutch, it is impossible to stop the output rotary member at a predetermined rotational position.
In recent years, electromagnetic spring-wound clutches of this kind have widely been used in the power transmission systems of electronic copying machines, etc. Electromagnetic spring-wound clutches for use with these apparatus are required to be compact in size as well as high in responsiveness of torque transmission. In addition, they should be constructed such that the output rotary member can be stopped after each one rotation, after each predetermined angle of rotation or after each predetermined number of rotations.
Electromagnetic spring-wound clutches which can comply with the above requirements have been proposed e.g. by U.S. Pat. No. 3,521,730 and U.S. Pat. No. 3,559,781. These clutches each comprise an input rotary member, an output rotary member, and a stop collar, which are disposed concentrically with each other, and further comprise a clutch spring engaged at its opposite ends by the output rotary member and the stop collar and disposed for gripping engagement with an outer peripheral surface of the input rotary member, a brake spring engaged at its opposite ends by the output rotary member and the stop collar, and disposed for gripping engagement with an outer peripheral surface of the output rotary member, an electromagnetic coil disposed opposite an outer peripheral surface of the stop collar, and a stop lever disposed for stopping rotation of the stop collar. The stop collar has its outer peripheral surface formed with a stop shoulder for engagement with an engaging end of the stop lever. When the stop lever is not operated, the clutch spring, which has its one end engaged by the output rotary member, grippingly engages the input rotary member to keep driving coupling of the input rotary member with the output rotary member. On the other hand, when the stop lever is operated, the engaging end of the stop lever engages the stop shoulder of the stop collar to prohibit rotation of the stop collar, and the resulting relative rotation of the stop collar and the output rotary member causes expansion of the clutch spring to release the driving coupling of the input and output rotary members, and simultaneously causes contraction of the brake spring into gripping engagement with the output rotary member to cause disengagement of the clutch and simultaneously stop the output rotary member at a predetermined position.
According to the proposed clutches, the output rotary member can be stopped at any desired rotational position. However, since the clutches each include more than one coil spring and a stop lever arranged at a location radially outward of the stop collar, they are inevitably large in axial size as well as in radial size, making it difficult to design them compact in size.